In conventional combustion plants a significant part of the energy content of the fuel is lost in the flue gases. This energy loss is reduced when the energy content is transferred to air, water, humidified air and the like, through condensation on the surfaces of heat exchangers. This technique is for instance applied for heating tap water.
In a steam boiler described in German Patent Specification No. 547 218, pre-heated air of combustion is humidified with heated pressurized water. The boiler waste gases also pre-heat air for combustion, boiler feed water and humidified air of combustion, in addition to pressurized water.
Swedish Patent Specification No. 163 470 describes a method and an arrangement of apparatus for heating humidified air and relatively cold air with the aid of the flue gases. The water used to humidify the air is optionally pre-heated, and the flue gases are then also used to this end.